1. Field of the Invention
The present invention relates to an antenna device for use in an indoor type radio station.
2. Description of the Related Art
FIGS. 9A and 9B are illustrations showing the positional configuration of a plurality of short patch antennas mounted in a conventional antenna device, wherein FIG. 9A is a front view and FIG. 9B is a side view. FIGS. 10A and 10B are magnified views respectively of the antenna device of FIG. 9A and FIG. 9B showing one of the short patch antennas mounted therein, wherein FIG. 10A is a sectional view observed from the line cut along Ixe2x80x94I of FIG. 9A, and FIG. 10B is a plain view of that short patch antenna. FIG. 11 is a block diagram showing a configuration of the conventional antenna device, and FIG. 12 is an illustration showing the characteristic of the radiation patterns of one of the short patch antennas (each of which may be referred to as an antenna element hereinafter) in the conventional antenna device. Further, FIG. 13 is an illustration showing the characteristic of the radiation pattern in the horizontal direction of six antenna elements closely disposed to one another in the conventional antenna device.
In these figures, reference numeral 100 denotes a rectangular ground plate, which is fixed along a wall surface within a house and supporting each of the short patch antennas whose details are explained later, numerals 101 to 106 are short patch antennas disposed on the ground plate 100 with a predetermined interval from one another, and numeral 107 denotes a covering stuff, which is 3.0 mm thick and made of a resin material such as ABS resin or the like.
In close proximity to the upper corners of the ground plate 100, a short patch antenna 101 and another short patch antenna 102, which are used for signal transmission, are disposed at a predetermined distance away from the edge surface of the ground plate 100 so as to prevent the reduction of the gains of these short patch antennas 101 and 102. On the other hand, in close proximity to the lower corners of the ground plate 100, a short patch antenna 103 and another short patch antenna 104, which are used for signal reception, are disposed at a predetermined distance away from the edge surface of the ground plate 100 or from each other so as to prevent the reduction of the gain of these short patch antennas 103 and 104, and also to eliminate the mutual effect on each other. Specially,the short patch antennas 103 and 104 provided as the signal reception antennas are disposed at a predetermined distance away from each other for eliminating the mutual effects, in order that they function together as a diversity antenna. Further, short patch antennas 106 and 105 are disposed between the short patch antennas 101 and 102 and also between the short patch antennas 103 and 104 respectively, each as an interference detection antenna for searching for a radio wave that can be an interference for communications of the base station.
Since the short patch antennas 101 to 106 have more or less the same configuration except their exact dimensions, the short patch antenna 104 is taken up here as an example for explaining the construction thereof.
The short patch antenna 104 is schematically composed of, as shown in FIGS. 10A and 10B, a radiation conductor portion 104a disposed in parallel to the surface of the ground plate 100 having a thickness of 1.6 mm at a predetermined interval therebetween, a ground conductor portion 104b in contact with the surface of the ground plate 100, and a bent portion 104c connecting these conductor portions 104a and 104b. The radiation conductor portion 104a is configured in such a manner as to be supplied with electric power by way of a supporting member 111 having an RF connector 110, whereas an insulation spacer 112 for maintaining the distance H between the radiation conductor portion 104a and the ground plate 100 is provided at the far end of the radiation conductor portion 104a. The ground conductor portion 104b is fixed to the ground plate 100 easily by a rivet 113. Note that only one side of the ground plate 100 is formed with a conductor pattern (not shown).
The length L1 of the radiation conductor portion 104a shown in FIG. 10B is determined in accordance with the frequency that the antenna uses, whereas the length L2 between the bent portion 104c and the power feeding point P is set in such a manner that the impedance becomes 50 ohm. The width W of the ground conductor portion 104b is determined by the gain of the antenna.
As shown in FIG. 11, the short patch antennas (TX1) 101 and (TX2) 102 are transmission antennas, which are connected respectively to a first transmitter 120 and a second transmitter 121. The short patch antenna (RX1) 103 is connected in a branched manner to a first receiver 123 and also to a third receiver 124 by way of an amplifier (AMP) 122, the short patch antenna (RX2) 104 is connected in a branched manner to a second receiver 126 and also to a fourth receiver 127 by way of an amplifier (AMP) 125. The short patch antenna (MX1) 105 is connected in a branched manner to the first receiver 123 and the second receiver 126 by way of an element 128 provided with the function of an amplifier and that of a frequency converter, and these first receiver 123 and second receiver 126 are connected to a first signal combiner 129. Further, the short patch antenna (MX2) 106 is connected in a branched manner to the third receiver 124 and the fourth receiver 127 by way of an element 130 provided with the function of an amplifier and that of a frequency converter, and these third receiver 124 and fourth receiver 127 are connected to a second signal combiner 131.
In the antenna device configured as mentioned above, the first transmitter 120 and the second transmitter 121 use different frequencies from each other, which are different also from those of the receivers 123, 124, 126 and 127.
The operation of the conventional antenna device is as follows.
First of all, when a signal transmitted from the nearby area of the radio station is received by the short patch antenna 105 that is an interference detection antenna, the thus received signal is amplified at the element 128 where a frequency conversion is processed, and thereafter sent to the first receiver 123 and the second receiver 126, wherein if the frequency of the thus received signal is same as that of the signals transmitted from the first transmitter 120 and the second transmitter 121, then the transmission of signals of the corresponding frequency is prohibited in order to prevent a possible interference.
After the above procedure by use of the interference detection antenna, a signal transmission at a usable frequency is started. In this case, a TDMA (Time Division Multiple Access) communication is enabled by dividing one cycle of a transmitted signal into three portions, and also by allocating one frequency to three communication lines. In this antenna device, two transmitters 120 and 121 are used, wherein if the both frequencies are usable; each transmitter can hold three communication lines, so that communications of 6 lines can be assured in parallel by the whole antenna device. The communication using this time-division method can be applied even in the signal receiving case.
Next, in the case of signal receiving, the same one signal is received simultaneously by two different antennas; namely the short patch antennas 103 and 104, and thereafter the thus received signals are amplified by the amplifier 122 and 125, respectively, and the amplified signals are then fed through the first receiver 123 and the second receiver 126 to a first signal combiner 129 where these signals are combined after synchronizing the phase of each signal. This can be done by use of the diversity technique for improving the strength of signal reception.
It is to be noted here that the radiation patterns made of one short patch antenna in the horizontal direction are made in such a manner, as shown in FIG. 12, that the level of the peak gain G1 of the radiation pattern of the main polarized wave, which is the gain obtained by directing the longitudinal direction of an antenna (hereinafter referred to just as the antenna direction) to the electric field, is almost same as that of the peak gain G2 of the radiation pattern of the cross-polarized wave, which is the gain obtained by directing the antenna to the direction intersecting the electric field at right angles. Contrary to this, in the case where a plurality of short patch antennas are closely disposed in a limited narrow area on the ground plate, the radiation patterns of one of those short patch antennas are made in such a manner that the level of the peak gain G2 of the cross-polarized wave is lower than that of the peak gain G1 of the main polarized wave by the effect of other short patch antennas which are disposed at a distance shorter than the length of 1 wavelength as shown in FIG. 13.
In this situation above, since in the case of an antenna device provided in an outdoor-type radio station, only the main polarized wave is generally used, so that no serious problem would occur even when the gain of the cross-polarized wave is lowered, whereas since in an antenna device provided in an indoor-type radio station, the transmitted/received waves crash against wall surfaces inside a house and subsequently the polarized surface is thus rotated, so that not only the main polarized wave but the cross-polarized wave can also be used for carrying out communications. For this reason, when the gain of the cross-polarized wave is lowered, the radiation pattern in the horizontal direction is deteriorated, degrading thereby the communication quality as a whole.
The present invention has been proposed to solve the problems aforementioned, and it is an object of the present invention to provide an antenna device which is capable of improving the radiation pattern in the horizontal direction of a plurality of short patch antennas which are closely mounted to each other.
In order to achieve the above object, an antenna device according to a first aspect of the present invention is constructed in such a manner that it comprises: a ground plate uprightly mounted in the vertical direction, a plurality of short patch antenna elements disposed on the ground plate adjacent to each other, and a pair of non-powered elements, each of which is disposed between the short patch antenna elements in such a manner as to extend in the vertical direction.
An antenna device constructed as above further comprises metal fixing elements for fixing the non-powered elements to the ground plate, which metal fixing elements protruding from the non-powered elements in the direction intersecting the direction of the electric field at right angles.
An antenna device according to further aspect of the present invention is constructed such that each of the non-powered elements are formed with a spacer at the respective end portions thereof for suppressing vibrations possibly transmitted from said ground plate, wherein the spacer is made of an electrically insulative material.
An antenna device according to further aspect of the present invention is constructed such that the metal fixing elements and the ground plate are electrically insulated from each other.
An antenna device according to still further aspect of the present invention is constructed such that the insulation in the above configuration is performed by slits formed in the surface of the ground plate around said respective metal fixing elements.